This invention relates to an electronic device for feeding an ion pump with two different tensions and for improved measuring of the pressure in the pump.
In an ion pump, the output current I is an almost linear function of the pressure P in the pump as expressed by the relation I=Kp.sup.n where K is a constant and n is generally between 1 and 1.2.
Thus, the ion pump can be used as a vacuometer in which the pressure value can be obtained from the current on the basis of the following relation: ##EQU1##
On the other hand, the current supplied by the feeder generally is the sum of the following contributions: I.sub.T =I+I.sub.D +I.sub.EC where I.sub.D is the leakage current of the feeder, cables and high tension connector, and I.sub.EC is the electronic current emitted by the cathode by field effect.
As I.sub.T is the only quantity that can be measured directly, and thus can be converted into pressure by the relation (1) recited above, the reading will only be reliable if and when the values I.sub.D and I.sub.EC are negligible relative to I as I.sub.T and I can thus be confused.
Now, as I.sub.D can be made negligible by adopting appropriate constructive measures, I.sub.EC varies in an unforeseeable manner with the life of the pump and can reach values of the order of 100 .mu.A, thus limiting the field of reliability of the pressure reading to about 10.sup.-6, as shown by the curve "a" in FIG. 1, which represents a typical current - pressure diagram at a feeding tension of 7 kV.
Therefore, the known ion pumps meet with a serious limitation when used as vacuometers as generally they are not reliable for measuring pressures below 10.sup.-6 torr.
On the other hand, as I.sub.EC varies, as said before, with the life of the pump, it is known to periodically reduce the field effect current by the hi-potting technique.
This technique consists in disconnecting the pump from its feeder to connect it to another feeder (hi-potter) capable of supplying a tension that is much higher than the normal tension, for example 15 kV.
It is evident that this technique is disadvantageous for most applications as it requires a manual intervention step with stopping of the pump for a relatively long time.
It is therefore an object of the present invention to eliminate or reduce the above-mentioned disadvantages of the known ion pumps and to provide an electronic feeding and measuring device which permits the ion pumps to be used as vacuometers for measuring pressures below 10.sup.-6 torr. and to be reliable at this measure up to pressures of above 10.sup.-10 torr.